Field of the Invention
In the art of injection molding of, for example, cylindrical bodies using a thermoplastic material, gating of the plastic into the mold cavity is conventionally accomplished at various positions about the cavity, for example, at the periphery, at an end offset from the cavity axis as well as on the axis itself. Gating at a location other than the center or on axis results in a flow pattern of molten material in filling the cavity along two or more paths which are physically separated. The material in following these paths eventually joins at finite locations thereby forming weld lines which in many instances can be visually perceived. By using axial or center gating, the injected material flows radially outwardly in all directions thereby filling the cavity uniformly, thereby avoiding the creation of such weld lines.
Plastic bonded magnets, including those which are cylindrical, are well known as disclosed and claimed in U.S. Pat. Nos. 3,842,148; 3,872,334 and 3,881,853. In these patents, gating is accomplished by a means of a sprue and gate located off-center of a cylindrical cavity which will, as just explained, produce a flow pattern resulting in weld lines. The magnet is formed of a mixture of thermoplastic powder and physically orientable, magnetizable particles such that by reason of the circuitous flow of the molten mixture in filling the cavity, a non-uniform alignment of magnetic particles within the cavity results as evidenced by a disparity in the geometric arrangement at the weld lines. As a consequence of this non-uniform arrangement, the magnetic properties of the magnet in cylindrical form will be correspondingly non-uniform about its periphery. In those instances in which uniform magnetic properties are required, off-center gating therefore should not be used.